Manufacture of electrical capacitors



Nov. 6, 1956 w. DUBlLlER MANUFACTURE OF ELECTRICAL CAPACITORS OriginalFiled June 25, 1951 INVENTOR W/l. 1. MM $05. /R

BY '70 74a ATTORNEY United States Patent M 'MANUFACTURE OF ELECTRICALCAPACITORS William Dubilier, 'NewRochelle, N. Y., assignor toCornell-Dubilier Electric Corporation, South Plainfield,

N. J., a corporation of Delaware Original application June 25, 1951,Serial No. 233,315. *Divided'and this application August 13, 1953,Serial 4 Claims. (Cl. 219-'19) This application is a division ofapplication Serial No. 233,315, filed June 25, 1951, entitled Means forand Method of Manufacturing Electrical Capacitors, now

In constructing wound paper'capacitors, it frequently happens that.particlesof dirt or'conducting matter are wound-in between the severallayers of paper ormetal foil in such a manner that the subsequentmanipulation of the capacitor for use will cause the particles topuncture the dielectric vbetween adjacent conducting surfacesand thusshort-circuit the capacitor. Furthermore, capacitor paper, usually madeof woodpulp, contains impurities or imperfections liable to causeorgiverise to injurious discharges and short-circuits, if a voltage isapplied to the terminals of the capacitor.

Such imperfections in the paper or dielectric are of an especiallyserious nature in so-called metallized paper capacitors, which compriseat least one layerof paper or an equivalent dielectric base coated onone or both sides with a thincoherent layer of conducting particlesdeposited thereon by evaporation or by any other process.

'In. practice, very thin paper is commonly used for capacitors-of thistype, having a thickness as low as .0004" or less and, .as a result, acapacitor having one or more paper layers initially contains a largenumber of shortcircuits or faulty spots, due both to the imperfectionsin the paper and produced by the deposition of the metal layer-orcoating process, as well as during the assembly of the unit.

If a capacitorwith a short-circuit or fault-as described has applied toit a voltage equal to or higherthan the operating'voltage, it willeither .be destroyed immediately by the short-circuit current-or agradual corona or other discharges will occur, resulting in afinalbreakdown or short-circuit and destruction of the capacitor after arelatively short operating period.

In view of the foregoing phenomenav and'difficulties, it has beencustomary in the past to clear capacitors, in

particular those of the single-paper and metallized paper type, ofshort-circuits or faulty spots in the dielectric by burning out theconducting or defective areas or paths in the paperdielectric betweenthe strips of metal foil or electrodecoatings, by applying a suitabledirect current to the terminals of the finishedimpregnated capacitor.

More particularly, in-the case of metallized paperca- Patented Nov. 6,1956 pacitors, using extremely thin metal layers or coatings depositedupon the paper or other dielectric base, application of a suitableshort-circuiting current will result in a burning or evaporation of thethin metal layer in the vicinity of the fault or short-circuit spot,without seriously affecting or burning the paper dielectric, in such amanner that the faulty spot or area will be separated by a sufiicientlylong insulating path from the electrodes under tension. A basicrequirement for such a burn-out or clearing of the capacitor ofshort-circuits or faulty spots, is the proper control of the burn-outenergy or-heat produced by the short-circuit or clearing current,'insuch a manner as to be effective in melting and evaporatingthe metallayer to a sufficient extent from the fault or shortcircuit point, whilebeing insufficient to materially affect or carbonize the paperdielectric, although some carbonization can not be'avoided in practice.

A similar consideration applies to capacitors using separate metallicand dielectric elements, especially singlepaper capacitors, although inthis case a clearing or burn-out of the short-circuit or defective spotsis more difiicult in view of the greater thickness of themetalelectrodes.

In the past, it has been customary in the manufacture of electricalcapacitors, especially those made with im- Such tests are usually madeto detect direct short-circuits,

but they'would not detectimperfect or defective spots in the dielectricitself, especially such weak spots which are dangerous at operatingvoltage in causing corona and similar discharges liable to result in agradual deterioration and final destruction of the dielectric andbreakdown of the capacitor after a relatively short operating period.

Moreover, testing of an impregnated unit in order to clearshort-circuits orweak spots in the dielectric, necessarily will resultin a deleterious effect upon the dielectric, i. e. both the paper andimpregnant, as it is practically impossible to so control the'burn-outenergy for the various dielectrics and impregnating materials anddifferent operating conditions existing in each case, as to efiectivelyclear the short-circuit or fault in the manner pointed out, withoutaffecting the dielectric in the vicinity of the short-circuit path. As aresult, a certain degree of carbonization of the paper and impairment ofthe 'impregnant in the neighborhood of the short-circuit can notbeavoided in practice.

This drawback assumes an especially serious nature in the case ofmetallized capacitors, in which case the number of short-circuits'orfaulty spots may be considerable, so as to result in a substantialchange of the final characteristics of the capacitor, due to themany'burnout or short-circuit points throughout the entire capacitor. Inparticular, carbonization of the paper and deterioration of theimpregnant near the short-circuit or faulty points, although negligiblewhen considering a single or limited number of burn-outs, whenconsidered for the entire capacitor having a considerable number offaults or burnouts, will result in an impairment of the electricalcharacteristics, in particular a decrease of the insulation resistanceof the finished capacitor.

As an example, an impregnated paper capacitor of standard design havingan initial insulating resistance of say 50,000 megohms will be reducedto 40,0000 megohms after being subjected to a first clearing or burnouttest by a voltage equal to or somewhat greater than the rated oroperating voltage of the capacitor, while this value may be furtherreduced to 30,000 megohms by a subsequent test and so forth, until thecapacitor finally breaks down completely and will be destroyed. The samephenomena occur in actual operation as a result of excess voltages,surges and other high tension phenomena, although spread over a longerperiod, thus greatly reducing the useful life of the capacitor.

An object of the present invention is to provide improved means formaking electrical capacitors using impregnated paper or an equivalentcomposite dielectric, whereby the capacitor may be cleared ofshort-circuits and faulty spots in the dielectric, substantially withoutaffecting its electrical characteristics, in particular its insulatingresistance and power factor.

Still another object is the provision of means for and a method ofmaking paper capacitors which will enable the attainment of uniformcharacteristics and result in a substantial reduction of the number ofrejects or defective units during manufacture.

A further object is the provision of a simple and reliable impulsetesting arrangment for clearing and testing electrical capacitors,especially of the metallized paper type.

The above and further objects and novel aspects of the invention will bebetter understood by the following detailed description considered inconjunction with the accompanying drawing, showing a circuit diagram ofa preferred form of testing system according to the invention.

The invention is especially suitable to the clearing of a woundelectrical capacitor before impregnation by a testing voltage or currentapplied to the electrodes in the form of periodic impulses of definiteamplitude and duration, to closely control the electrical burn-outenergy. In this manner, by testing the un-impregnated condenser, anyphysical or chemical changes of the dielectric and impregnant caused bythe testing or clearing current is avoided, while the use of currentimpulses for effecting the clearing or burn-out provides a simple andeflicient means to limit and control the burn-out energy so as to bemost effective in clearing the short-circuits and faults in thedielectric, substantially without regard to the paper dielectric andimpregnating medium later to be introduced by the impregnating process.As a matter of fact, the effect of any carbonization or otherundesirable change of paper dielectric resulting from the burn-outcurrent, will be substantially minimized or overcome by the subsequentimpregnation of the capacitor by a suitable insulating material, such aswax, oil or a swathetic com pound well known in the art.

The testing or burning out of the capacitor in its wound butun-impregnated state by closely controlled amounts of burn-out energybursts or pulses, has the further advantage of containing or restrictingthe buming-out process to the short-circuit or defective spots, on theone hand, and enabling a uniform and simultaneous clearing of the entiredielectric of the capacitor by a single operation in a most eflicientand reliable manner.

In other words, the capacitors according to the invention are subjectedto an accelerated life test as far as the burn-out or self-healing ofthe short-circuits or faults are concerned, such tests, however, beingcarried out under conditions difien'ng from the actual operatingcondition, viz. in the un-impregnated state of the capacitor,

to substantially avoid the previous defects and drawbacks inherent inthe self-healing or burn-out process, in particular the reduction of theinsulation resistance and power factor of the capacitor.

In carrying out the invention, a single paper provided with the propermetallic layer or coatings thereon or a number of separate foil andpaper strips are wound into a convolute capacitor unit in accordancewith standard practice. Instead of these units being impregnated as hasbeen the custom, they are connected to a testing voltage above theoperating voltage for a short period of time, preferably by connectingthe unit to a charged storage capacitor to provide a measured amount ofburnout energy and to closely control the clearing of the faults,substantially without carbonization of the dielectric.

The accompanying drawing shows a circuit diagram of a preferredautomatic testing system according to the invention.

In the drawing, the numeral 10 represents an adjustable storagecapacitor connected to a source of direct current indicated by theterminals 11 and 12 in series with a high-ohmic resistor 13. A furthershunt circuit connected across the capacitor 10 comprises thecathodeanode path of a gaseous discharge tube or thyratron 14 in serieswith the capacitor to be tested, to be connected between terminals 15and 16. The thyratron 14 may be of standard construction comprising acathode heated to proper electron emitting temperature from analternating current source through a heating transformer 17, a grid orcontrol electrode and a plate or anode. The grid is normally biasednegatively with respect to the cathode just below the critical orbreakdown potential of the tube, the normal or steady bias havingsuperimposed thereon an alternating control voltage, to periodicallyraise or lower the grid potential above and below the critical value ata given rate or frequency. Capacitor 10 may be a single variable unit ora bank of capacitor units connected in parallel.

In the example shown, the steady grid bias for the thyratron 14 isderived from the output potentiometer or voltage divider of a rectifierpower supply comprising a pair of resistors 22 and 23 connected inseries. The rest of the power supply circuit shown is of standardconstruction and comprises an input transformer 25, doublewave rectifier24 and smoothing filter 26 to convert the alternating voltage into asmooth or steady direct current voltage across the potentiometer 22, 23.In order to provide a negative grid bias potential for the thyratron 14,the grid of the latter is connected to an intermediate point on thepotentiometer resistor 23 near the low voltage or ground side of thecircuit, the cathode of the thyratron being connected to the junctionbetween the resistors 22 and 23 or a point which is positive in respectto the grid potential.

The alternating current control voltage superimposed upon the steadygrid bias voltage is produced, in the example shown, by means of a gastube or relaxation oscillator energized by the potentiometer 22, 23 andcomprising a high ohmic resistance 21 in series with a capacitor 18, thelatter being shunted by a two-element gas discharge tube 20. In acircuit of this type, the condenser 18 is charged slowly through theresistance 21 until reaching a potential equal to the ionization orbreakdown potential of the tube 20, thus causing a breakdown of thelatter anddischarge of the condenser. The condenser is then chargedagain and the cycle repeated, thus resulting in the generation of analternating current or voltage of a frequency determined by the size ofthe capacitor 18 and the value of the resistance 21. This voltage isapplied to the grid and cathode of the tube 14 by way of a transformer27.

In testing or clearing a condenser connected to the terminals 15 and 16,the operation of the circuit described above is as follows: Assumingthat the capacitor 10 has been charged to the full voltage of thevoltage testing source connected to terminals 11 and 12, and, providedfurther that the steady potential on the grid of the tube 14 is lessthan the critical or breakdown potential, a low voltage gas discharge orarc will be initiated through the tube during a positive half cycle ofthe control voltage supplied by the relaxation oscillator 18, 20, 21,causing initiation of a low voltage ionic discharge and a current toflow through the tube and one or more of the short-circuit points of thecapacitor under test connected between terminals 15 and 16.

The capacitor 10 is so designed as to perform the double function ofproviding limited amounts of electrical energy suflicient to clear theshort-circuit points of a capacitor under test, while substantiallypreventing car- .bonization or other deterioration of the paperdielectric,

ii I) on theone hand, and to serve as a means for'interrupting thedischarge through the tube 14 after its chargelias -beenreduced to apoint insufficient to maintain an ionic discharge current through thetube. The resistor 13 is so designed as to result in a slow or gradualcharging of the capacitorlO, on theone hand, and to prevent a directdischarge of the source voltage through the tube 14".- After. thecapacitor has been discharged, the tube 14-will be extinguished, thusinterrupting the current through the capacitor under test. The sourcevoltage is unable to maintain or reestablish the discharge through thetube 14 on account of the limiting resistance 13, so that the capacitor10 is gradually recharged to initiate a new testing cycle the frequencyof the spontaneous testing cycles thus obtained depending upon the timeconstant of the storage capacitor 10 and series resistor 13.

In order to more accurately determine the charge and discharge ofcapacitor 10 and to insure its being charged to the full testing voltageduring each testing cycle, the tube 14 is periodically renderedconducting and nonconducting by the effect of the alternating controlvoltage of the relaxation oscillator, in the manner described before.For this purpose, the time constant of resistance 13 and capacitor 10 ischosen to be less than the time constant of resistance 21 and capacitor18 of the relaxation oscillator. In other words, the frequency of thecontrol impulses applied to the grid of the thyratron 14 should be lessthan the spontaneous charging .and discharging frequency of the storagecapacitor 10.

3 Accordingly, a series of short-clrcuiting or clearing current pulsesof definite amplitude and frequency are applied to the capacitor undertest, as indicated by the gas discharge through the tube 14 in the formof luminous flashes. The clearing current pulses and luminous flasheswill continue until the impedance of the capacitor under test hasreached a suificiently high value to cause the anode potential of thetube 14 to fall below the critical or firing potential for a given gridbias comprising the steady bias and the periodic bias voltage suppliedby the relaxation oscillator 18, 20, 21. This makes it possible by theproper choice of the tube parameters and/or bias potentials toautomatically clear a capacitor to have a desired insulating resistanceor impedance, substantially without measurement or comparison withstandard devices. As soon as all the short-circuits through thecapacitor under test have been cleared in this manner, the tube willcease to flash or operate, thus apprising the operator that thecapacitor has been substantially cleared of short-circuited and faultyspots.

If the dry and unimpregnated capacitor were tested by continuouslyconnecting it to a higher testing voltage or to the standard testvoltage for the finished condenser, it Would result in a quick breakdownand carbonization of the dielectric material, the breakdown occurring atthe weakest spot in the dielectric. By the method above described, thebreakdown current is of such a short duration or the burn-out energyclearly limited and applied in properly timed sequence to merely causethe burning away of the metal around the defective spots or shortcircuits, but being of insufficient duration to carbonize or otherwisematerially injure the paper or other dielectric material.

If, nevertheless, a slight carbonization of the paper or dielectricmaterial near the short-circuit point should occur, due to variations inthe dielectric or lack of close control or dissipation of the heatproduced by the burnout current, this will not have any injurious effecton the capacitor, as the unit will be subsequently thoroughlyimpregnated with an insulating compound, whereas with the old method oftesting the impregnated unit, in addition to the effect of the burn-outcurrent on the impregnant, the carbonized particles will remain andcause the capacitor to become defective after a relatively shortoperating period.

It: is thus possible by the use of the invention to test a capacitorhaving a rated or operating voltage of say 200'volts at a voltage of300'volts' in its dry and impregnate'd' condition- If this capacitor,after testing, is then impregnated with an insulating compound and usedon anoperating voltage of 200 volts, it will last almostindefinitely,and can, if desired be used with higher operating voltages for aconsiderable length of time and without substantial variation of itselectrical characteristics, in particular the insulating resistance andpower factor of the capacitor.

In the foregoing, the invention has been described with reference to .aspecific illustrative device. It will be evident, however, thatmodifications, as well as the substitution of equivalent parts orcircuits for those disclosed for illustration, may be made withoutdeparting from the inventive concept herein disclosed and defined in theappended claims, and it is desired, therefore, that the specificationand drawing be regarded in an illustrative rather than a limiting sense.

I claim:

1. Apparatus for automatically increasing the leakage resistance ofelectrical capacitors by clearing inter electrode short circuits andfaults therein comprising a storage capacitor, a charging circuitconnected across said storage capacitor including a direct currentvoltage source connected in series with a charging resistor, a shuntdischarge circuit connected across said storage capacitor including acapacitor to be cleared and a gaseous discharge tube disposed in serieswith said last-mentioned capacitor and responsive to the state of chargeof said storage capacitor and the leakage resistance of said capacitorto be cleared for sequentially and automatically opening and closingsaid shunt discharge circuit.

2. Apparatus for automatically increasing the leakage resistance ofelectrical capacitors by clearing inter electrode short circuits andfaults therein comprising a storage capacitor, a charging circuitconnected across said storage capacitor including a direct currentsource connected in series with a charging resistor, a gaseous dischargetube having a pair of main electrodes and a control electrode, a shuntdischarge circuit connected across said storage capacitor including acapacitor to be cleared disposed in series with the main discharge pathof said tube, means for generating a periodic control potential having afrequency less than the spontaneous charging and discharging frequencyof said storage capacitor and means applying said periodic controlpotential to the control electrode of said tube for permittingcontinuing repetitive ionic discharge of said tube and consequentdischarge of said storage capacitor through said capacitor to be cleareduntil the leakage resistance of said lastmentioned capacitor increasesto such a value as to prevent ionic discharge of said gaseous dischargetube.

3. Apparatus for clearing short circuits and faults in electricalcapacitors comprising a storage capacitor, a charging circuit connectedacross said storage capacitor including a source of direct currentvoltage in series with a high-ohmic charging resistor, a gaseousdischarge tube having a pair of main electrodes and a dischargeinitiating control electrode, a discharging circuit also connectedacross said storage capacitor including the main discharge path of saidtube in series with a capacitor to be cleared, means for applying asteady bias potential to said control electrode close to and below thebreakdown potential of said tube, and a relaxation oscillator comprisinga further resistor in series with a further capacitor shunted by agaseous discharge tube to produce a periodic control potential, means toapply said control potential to said control electrode superimposed uponsaid bias potential to periodically raise said bias potential above thebreakdown potential of said tube, the time constant of said furthercapacitor and resistor being in excess of the time constant of saidstorage capacitor and charging resistor.

4. A device for clearing short circuits and faults in electricalcapacitors comprising a storage capacitor, a charging circuit connectedacross said storage capacitor including a source of direct currentvoltage in series with a high-ohmic charging resistor, a gaseousdischarge tube having a pair of main electrodes and a dischargeinitiating control electrode, a discharging circuit also connectedacross said storage capacitor including a capacitor to be cleared inseries with the main discharge path of said tube, and means for applyinga periodic control poten- 1 References Cited in the file of this patentUNITED STATES PATENTS Katzman Feb. 9, 1937 Sidney Nov. 7, 1939

